Mining and Refining: Sulfur


When you think of the periodic table, some elements just have a vibe to them that’s completely unscientific, but nonetheless undeniable. Precious metals like gold and silver are obvious examples, associated as they always have been with the wealth of kings. Copper and iron are sturdy working-class metals, each worthy of having entire ages of human industry named after them, with silicon now forming the backbone of our current Information Age. Carbon builds up the chemistry of life itself and fuels almost all human endeavors, and none of us would get very far without oxygen.

But what about sulfur? Nobody seems to think much about poor sulfur, and when they do it tends to be derogatory. Sulfur puts the stink in rotten eggs, threatens us when it spews from the mouths of volcanoes, and can become a deadly threat when used to make gunpowder. Sulfur seems like something more associated with the noxious processes and bleak factories of the early Industrial Revolution, not a component of our modern, high-technology world.

And yet despite its malodorous and low-tech reputation, there are actually few industrial processes that don’t depend on massive amounts of sulfur in some way. Sulfur is a critical ingredient in processes that form the foundation of almost all industry, so its production is usually a matter of national and economic security, which is odd considering that nearly all the sulfur we use is recovered from the waste of other industrial processes.

It’s Always Oil

Sulfur is one of those elements that’s remarkably abundant in the universe and while it does occur in its elemental state, it’s more typically found as a compound with something else. This is thanks to sulfur’s ability to form more than 30 allotropes, or different forms in the same physical state, and to the wide range of chemical reactions it participates in — there’s a sulfide or sulfate of almost every other element on the periodic table, except for those snooty Noble gases.

On Earth, sulfur is usually found in sulfide minerals, where an atom with a positive charge binds with one or more negatively charged sulfur atoms. Examples include chalcocite (copper sulfide), galena (lead sulfide), cinnabar (mercury sulfide), and pyrite (iron sulfide). Sulfates, where sulfur and oxygen bind with a cation, are also common; the gypsum used to make drywall boards and PVC pipes is calcium sulfate, for example.

The abundance of sulfide and sulfate minerals, and the fact that generally whatever the sulfur is bound to in these minerals is valuable in its own right, means that sulfur can be recovered as a byproduct of smelting operations, particularly from smelting of lead, copper, and zinc ores. We’ve covered copper smelting in some depth; the basic process is the same for most sulfide mineral smelting, and uses heat to drive off the sulfides. In less environmentally aware times, and when there were other, cheaper sources of sulfur, the sulfur-laden flue gases were just vented off, leading to a series of reactions in the atmosphere that culminated in sulfuric acid falling from the sky — acid rain.

Recovery of sulfur from smelter flue gas is just a small fraction of current sulfur production, though — only about 7% in the USA right now. The majority of sulfur production worldwide comes from either petroleum refining or natural gas production, where sulfides are contaminants that need to be removed. Cleaning up sulfides from “sour” gases — so-called because they are both acidic and smelly thanks to hydrogen sulfide (H2S) — is the job of an amine treater, or sweetener. Amine treaters are used in all sorts of industrial processes; we ran into them back when we discussed how helium is refined from natural gas. Amine treatment relies on the ability of amine solutions, like monoethanolamine (MEA) and diethanolamine (DEA) to react with the acid gases, like H2S and CO2, and make them more soluble in the scrubbing solution than in the process gas. The sulfide-rich amine solution is then boiled to strip the sulfides off and regenerate the amine for reuse. The process renders the incoming sour gas clean enough to release into the atmosphere, as well as a supply of H2S, which can then be processed into elemental sulfur.

[embedded content]

Sour to Sweet

The hydrogen sulfide that’s stripped out of the sour gas is very toxic to humans, and is particularly dangerous because at sufficiently high concentrations, it paralyzes olfactory nerves; people exposed to it for just a few minutes think that the gas has dispersed and the danger is gone because they can’t smell the rotten-egg stink anymore. Although it has some industrial uses, most hydrogen sulfide is converted to elemental sulfur, which is much easier to store and transport. The main process used to convert H2S to elemental sulfur is the Claus process, named after German chemist Carl Friedrich Claus, who invented it in 1883.

The Claus process is a two-part process: a thermal step, where hydrogen sulfide is burned in an oxygen atmosphere, and a catalytic step that boosts sulfur yield. The thermal step is extremely exothermic and takes place inside what’s known as a Claus furnace, which is a strong chamber lined with refractory material to withstand temperatures in excess of 1,050°C, which are needed to burn off unwanted products that will clog up the downstream catalyst bed. The overall reaction of the thermal step looks like this:

bf 2 H_{2}S + 3 O_{2} rightarrow 2 SO_{2} + 2 H_{2}O

bf 2 H_{2}S + SO_{2} rightarrow 3 S + 2 H_{2}O

Because of the high temperatures inside the Claus furnace, the sulfur produced by the thermal step is a vapor. The thermal step is responsible for the bulk of sulfur production, about 60-70%. To increase the yield, the sulfur-rich vapor from the thermal step is fed into a series of reheaters and catalytic converters. The reheaters are used to make sure the sulfur vapor doesn’t condense into a liquid, while the remaining hydrogen sulfide and sulfur dioxide from the thermal step react on mixed beds of alumina and titania catalyst to produce more sulfur vapor, along with water, basically by repeating the second step of the reaction above to wring the last little bit of sulfur out of the feedstock. The tail gas from the sulfur recovery unit (SRU), as the Claus process equipment is collectively known, still needs scrubbing before being released, but in general, about 95 to 99.9% of the sulfur in the feedstock is recovered as elemental sulfur.

Forbidden Lemon Drops

Up to this point, all the processes used have been at high enough temperatures that the elemental sulfur has been in the gaseous phase. But at this point, condensing the vapor out into a liquid makes it easier to handle. Sulfur is a viscous, dark red-orange liquid at 125°C, a temperature that’s easy to reach and maintain industrially, meaning that liquid sulfur can be pumped around plants in heated, insulated pipes. Liquid sulfur can even be shipped short distances in insulated tankers, but to store and transport a lot of sulfur, it has to be converted back into a solid.

Solid sulfur is quite easy to make. Hot liquid sulfur is pumped into a machine called a rotoformer, which is basically a big perforated cylinder. The liquid sulfur flows out of the holes as the cylinder rotates and gets extruded out onto a steel conveyor belt as little liquid dots. Water sprayed on the underside of the belt cools the sulfur, which solidifies into little yellow bits that look like lemon drops. In fact, these little nubbins of sulfur are called “pastilles,” in a nod to their confectionary look. A rotoformer line can make many tons of pastilles a day, and the sulfur is piled up into mountains before being loaded onto bulk cargo ships or trains for shipment.

[embedded content]

The King of Chemicals

But what’s the use of all this stuff? Elemental sulfur has a lot of industrial uses — vulcanization of rubber for tires comes to mind — but the majority of sulfur is turned into a single, immensely useful product: sulfuric acid. About 256 million tonnes of sulfuric acid were made in 2020; some estimates put future demand at 400 million tonnes annually. Most sulfuric acid goes directly into fertilizer manufacturing, where it is used to dissolve phosphate minerals into phosphoric acid, the feedstock for phosphate fertilizers. Sulfuric acid is also used to make dyes, pharmaceuticals, plastics, inks, explosives, and, of course, car batteries. It’s known as “The King of Chemicals” for very good reasons.

Sulfuric acid is made in a process that resembles a reverse version of the reactions used to remove it from natural gas and crude oil. There are two main processes, the contact process and the wet sulfuric acid process. Both are very similar and start with burning elemental sulfur in an oxygen atmosphere to create sulfur dioxide (SO2), and then continuing the oxidation of the products by passing it over a catalyst of vanadium(V) oxide. This adds another oxygen and makes sulfur trioxide (SO3), which is then converted to sulfuric acid, or H2SO4:

bf S + O_{2} rightarrow SO_{2}

bf 2 SO_{2} + O_{2} rightleftharpoons 2 SO_{3}

bf SO_{3} + H_{2}Orightarrow H_{2}SO_{4}

Sulfur Sans Carbon?

Sulfuric acid’s royal status in the chemical world is not just an honorific — it really is an indication of the industrial might of a nation. Without sulfuric acid, most of the industrial processes in the world would quickly grind to a halt, leaving humanity hungry, naked, sick, and without any clean water. So a continued supply of it, and therefore of sulfur, is critical to keeping life as we’ve come to know it running smoothly.

But, because sulfur production has become so tightly meshed into fossil fuel production, we’re potentially facing a future where sulfur becomes scarce thanks to decarbonization. There were methods for extracting sulfur before the oil industry made sulfur essentially a free byproduct; the Frasch method used high-pressure steam injected into boreholes dug into natural formations where ancient microbes reduced environmental sulfur and left huge deposits of elemental sulfur. But this method is much more expensive than current sulfur recovery methods are, and has a high environmental cost that might be hard to swallow.

One thing is for sure, though: for modern industrial society to continue, the sulfur must flow. How it gets extracted safely and cheaply in a decarbonized world will be an interesting engineering challenge.

Latest Intelligence